"Last time you had a bad cold, you likely had less energy than usual. You lay around and didn't have any enthusiasm for your usual activities. After it dragged on for a day or two, a sense of helplessness probably set in. It was hard to remember what feeling good felt like or how you could ever bound off the couch again.
In short, for a few days, you probably felt a lot like someone with depression.
And increasingly, scientists think it's no coincidence that a mental illness feels like a physical one.
A growing body of research on conditions from bipolar disorder to schizophrenia to depression is starting to suggest a tighter link than was previously realized between ailments of the mind and body. Activation of the immune system seems to play a crucial role in both."